[srslte-users] srslte_pss_synch_find_pss returns peak position greater than the subframe size

Andre Puschmann andre.puschmann at softwareradiosystems.com
Mon May 15 12:16:51 UTC 2017


Hey Felipe,


On 11.05.2017 19:34, Felipe Augusto Pereira de Figueiredo wrote:
> Dear srsLTE group,
> 
> I'm studying the srsLTE code and I`m confused by one thing I've found out.
> 
> I observed that the function: srslte_pss_synch_find_pss from pss.c
> sometimes returns a peak position that is greater than the subframe,
> i.e., 5760 in the case of 5 MHz and using the sampling rate of 5.76 MHz.

During the initial find phase, we capture 5 subframes at once to make
sure the samples contain one PSS sequence. Note that the PSS is
transmitted twice in every frame, i.e. every 5ms. So running the PSS
search on this set of samples may return a PSS position that is _after_
the first subframe, since it is not known where exactly the PSS started
(otherwise we would already be in sync).

Hope that helps.

Cheers
Andre



> 
> I've checked and that function uses convolution FFT to do the
> correlation, see below:
> conv_output_len = srslte_conv_fft_cc_run(&q->conv_fft, q->tmp_input,
> q->pss_signal_time[q->N_id_2], q->conv_output);
> 
> If the peak is used to align the subframe how can it be greater than the
> subframe? Is this behavior correct?
> 
> Thanks in advance and Best Regards,
> 
> Felipe Augusto
> 
> 
> 
> 
> 
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